SURFACE MOUNT SOLDER ASSEMBLY OF LEADLESS INTEGRATED CIRCUIT PACKAGES TO SUBSTRATES

摘要

Described are a process for soldering at least one component having solder bumps to a substrate and a process for forming solder bumps on metal pads of an element, such as an IC package or substrate or both. The bumps are formed by stencil printing solder paste deposits on the metal pads, heating the solder pas te deposits to reflow temperature of the solder in the solder paste deposits, and a llowing the molten solder in each deposit to coalesce and during subsequent cooling soli dify forming the bumps on the metal pads. The bumps are formed by conducting the stencil printing through apertures in an ultra-thick stencil, the apertures havi ng trapezoidal crossection in the plane normal to the broad surfaces of the stencil with the top opening being smaller than the bottom opening and with the walls of the aperture sloping at an angle within a range of from 1 to 45 degrees from the ver tical, the solder paste having a low tackiness and high metal loading, and the solder p aste deposits covering an area which is equal to or exceeds an area of the metal pad in any ratio between 1.5:1 and 5:1. Bumps formed in this manner lead to the formati on of reliable solder joints.